Inorganic complexing agents and organic complexing agents are well-known and have been used extensively in numerous industrial applications. Generally, complexing agents are used either to help remove metal ions from solution or to help solubilize metal ions in solution. The variety of complexing agents that have been developed demonstrates that no one particular complexing agent or class of complexing agents has performed satisfactorily across all industries to remove and/or solubilize all metal ions in all applications. Therefore, specific complexing agents were developed to solve particular industrial application problems. However, these specific metal complexing agents, in general, have the disadvantage of small effective pH range, persisting in the environment and/or of adversely affecting the environment. Most cannot be used in very acid media such as media in the pH range of 2 or less.
A known class of efficient chelating agents are the phosphonic acids and diphosphonic acids, generally characterized by the structural formulas, RCH.sub.2 PO.sub.3 H.sub.2 and RCH (PO.sub.3 H.sub.2).sub.2, wherein R is an alkyl, aryl, substituted alkyl, or substituted aryl group. The phosphonic acid and diphosphonic acid compounds, although effective as complexing agents, also possess disadvantages. One particular disadvantage is their excellent thermodynamic stability. Although thermodynamic stability normally is desirable in industrial compounds, such stability is a definite disadvantage after the complexing action is no longer desirable as when recovery of the complexed metal from solution is required.
Complexing agents are useful, for example, for clearing waste effluent streams from industrial processing and manufacturing. Another example is the case where one or more environmentally-damaging and/or toxic metal ions, such as radioactive metal ions or metal ions such as cadmium or zinc are intermixed with one or more non-damaging and/or non-toxic metal ions. Selective removal of the cadmium or zinc reduces disposal costs. A further example is the situation where a waste stream contains several different metal ion species wherein only one or two of the metal ions species are sufficiently valuable to justify separation of the metal ion from the waste stream for eventual isolation, regeneration and reuse. In each case, it is desirable to be able to remove selected metal ions and recover them or concentrate them for disposal.
Among the most objectionable byproducts is radioactive waste. The most toxic constituents in radioactive waste are the highly radioactive transuranic elements, i.e., the man-made elements heavier than uranium, that have extremely long half-lives and keep nuclear wastes toxic for millions of years. Currently, disposal of such highly toxic radioactive wastes requires converting the radioactive waste into a glass via vitrification, then burying the glass in deep geologic mines or repositories. Millions of gallons of waste that contain transuranic elements are currently awaiting such a disposal treatment.